In operating cryogenic refrigeration equipment, effort must continually be expended to minimize the amount of air that enters the equipment during operation. In such equipment, the refrigerant is a cryogenic fluid that is vaporized during the refrigeration process. Characteristically, air manages to enter the refrigeration enclosure through ports which allow product to pass into and out of the enclosure. Typically, the air is much warmer and contains a considerable amount of moisture relative to the environment inside the enclosure. Moreover, the moist air can be thought of as a contaminant in the sense that it reduces the purity level of the vapor inside the enclosure.
There are a number of reasons to minimize air infiltration: refrigeration efficiency, economics and capacity to recycle the vaporized refrigerant. Refrigeration efficiency is defined as the quantity of heat removed from a product being cooled, compared to the amount of refrigeration being expended by the cryogen. When moist air enters a refrigeration enclosure, it will necessarily be cooled to the current temperature therewithin. Cooling of air instead of product decreases the cooling potential of the refrigerant, and hence decreases refrigeration efficiency. Additionally, freezing of the water vapor can potentially lead to a damaging build-up of ice inside the enclosure. Ice build-up can become severe enough to require a stop in the production line of the product being cooled, to allow for a thawing period. Clearly, the cooling potential of the refrigerant is reduced or lost during this thaw cycle. The net result is a higher cost in operation.
To recycle the refrigerant, the best approach is to start with the purest stream possible from the source, which in this case is a refrigeration enclosure. The economics associated with recycling a refrigerant are greatly impacted by relatively small changes in the purity of the vapor inside the enclosure. Hence, the greatest economic advantage is achieved when air infiltration is minimized.
Minimization of air infiltration is important in both tunnel and spiral refrigerators. Typically, a tunnel refrigeration enclosure has an entrance port to allow product to enter the enclosure, an exit port to allow product to leave the enclosure, and a flat conveyor in-between. A spiral refrigeration enclosure has similar porting, except that the ports are at different heights relative to the base of the refrigeration enclosure. Inside the enclosure, the conveyor follows a spiral or helical pattern between the ports.
U.S. Pat. No. 3,728,869 to Schmidt describes the recycling of cryogenic vapors from an enclosure (primarily a spiral refrigerator). The pressure within the refrigeration enclosure is kept above atmospheric pressure to minimize air and other contaminant infiltration, and pressure and gravitational effects cause a flow thereof from each refrigeration port. The exiting vapor is collected in adjacent vestibules or spillover boxes in such a manner as to form a vapor barrier above the vestibule. Air infiltration is prevented by a vapor dam. Vapor is removed from the bottom of a vestibule by a piping network driven by a blower system. Control of vapor removal is through motorized on/off dampers in the ducting leading away from the vestibules.
U.S. Pat. No. 4,356,707 to Tyree et al., describes several refrigeration enclosure designs which utilize both mechanical and cryogenic refrigeration. A spiral refrigeration enclosure using a cryogenic refrigerant is described wherein diluting chambers are positioned adjacent the refrigeration ports. The concern at a lower port is to minimize outflow of the denser-than-air cryogen vapor from the refrigeration enclosure. A chamber adjacent to the lower port includes several baffles and a blower system operated at a constant frequency. Vapor is retarded from leaving the refrigeration enclosure by sucking a portion of the vapor from a dilution chamber and redirecting it back into the enclosure. The remaining portion of the vapor exits through the refrigeration enclosure opening and dilutes any air trying to enter the enclosure. Side vanes, manually positioned, are used to balance flow across a conveyor belt.
Variable fan speed control has been employed in the prior art as a means to prevent premature spillover of cryogenic vapor from a refrigeration enclosure or to prevent air from entering. In U.S. Pat. Nos. 4,528,819 (Klee) and 4,800,728 (Klee), the concern is how to prevent loss of cryogen vapor from a refrigeration enclosure or air infiltrating into the enclosure. A temperature sensor is used to indicate whether cryogenic vapor is leaving the enclosure or air is entering the enclosure. Coupled to the temperature sensor is a blower system. In U.S. Pat. No. 4,528,819, the blower is on the exhaust line of the refrigeration enclosure. In U.S. Pat. No. 4,800,728, the blower mechanism is internal to the refrigeration enclosure and is part of the circulation system.
Other methods have been employed to minimize the vapor leaving a refrigerator or the surrounding air from contaminating the interior of a refrigeration enclosure. U.S. Pat. No. 4,947,654 (Sink et al.) describes atmosphere control within spiral refrigerators and tunnel refrigerators. For spiral refrigerators, an improvement to the dilution system discussed in U.S. Pat. No. 4,356,707 is disclosed. The blower or blowers of the dilution system are no longer operated at a fixed frequency, but control of the blower system is now coupled to the cryogen injection rate. The primary sensing device can be either a temperature sensor inside the enclosure or a pressure sensor in the liquid supply line that feeds the cryogen injectors. The exit port can have a similar system to prevent air infiltration by letting a small amount of vapor exit through the port. For a tunnel refrigerator, similar means are discussed for minimizing air infiltration and reducing the premature loss of vapor from the refrigerator.
U.S. Pat. No. 4,955,206 (Lang et al.) discusses a variable speed control method for maintaining the environment within a refrigerator. For a tunnel refrigerator, maintenance of the internal environment is enhanced by the addition of a photocell transmitter and receiver sensor system located outside the entrance port and a baffle-linkage scheme surrounding one of the internal axial fans. The sensor system provides control information based upon how much vapor is leaving the refrigerator. If an excessively high level of vapor is escaping, the baffle-linkage system directs flow away from the port. If the opposite is true, the baffle-linkage responds by directing vapor toward the port. In a spiral refrigerator, the dilution blower system is coupled to the photocell sensing system and is not dependent on the injection rate. In both refrigerator configurations, the blower systems have either variable or single speed drives.
Another method for maintaining cryogenic purity inside a refrigeration enclosure is by employing a controlled evacuation system on the enclosure. U.S. Pat. No. 5,186,008 (Appolonia et al.) discusses a method for controlling an amount of vapor extracted from an enclosure as part of a recycle effort. For a spiral refrigeration enclosure, the locations of suction are at an upper vestibule and at the bottom of the refrigeration enclosure. For the bottom suction location, the amount of vapor leaving the enclosure is a constant ratio relative to the injection rate. The remaining portion of vapor resulting from injected cryogen exits through the entrance and exit ports. Sufficient suction needs to be applied at the upper vestibule to minimize gravitational effects on the vapor flow leaving through the lower port and to prevent air infiltration in the upper port. Hence, the pressure in the upper vestibule region is required to be the lowest pressure relative to the refrigeration enclosure and the surrounding atmosphere.
It is an object of this invention to provide an improved apparatus and method for minimizing escape of refrigerant vapor from a refrigeration enclosure and inlet of air into the enclosure.